Kinetics of changes in the crypts of the jejunal mucosa of dimethylhydrazine-treated rats

Assessment of the mode of action underlying development of rodent small intestinal tumors following oral exposure to hexavalent chromium and relevance to humans

Abstract Chronic exposure to high concentrations of hexavalent chromium (Cr(VI)) in drinking water causes intestinal adenomas and carcinomas in mice, but not in rats. Cr(VI) causes damage to intestinal villi and crypt hyperplasia in mice after only one week of exposure. After two years of exposure, intestinal damage and crypt hyperplasia are evident in mice (but not rats), as are intestinal tumors. Although Cr(VI) has genotoxic properties, these findings suggest that intestinal tumors in mice arise as a result of chronic mucosal injury. To better understand the mode of action (MOA) of Cr(VI) in the intestine, a 90-day drinking water study was conducted to collect histological, biochemical, toxicogenomic and pharmacokinetic data in intestinal tissues. Using MOA analyses and human relevance frameworks proposed by national and international regulatory agencies, the weight of evidence supports a cytotoxic MOA with the following key events: (a) absorption of Cr(VI) from the intestinal lumen, (b) toxicity to intestinal villi, (c) crypt regenerative hyperplasia and (d) clonal expansion of mutations within the crypt stem cells, resulting in late onset tumorigenesis. This article summarizes the data supporting each key event in the MOA, as well as data that argue against a mutagenic MOA for Cr(VI)-induced intestinal tumors.

Ethylene diamine tetraacetic acid induced colonic crypt cell hyperproliferation in rats

AIM: To investigate the effect of ethylene diamine tetraacetic acid (EDTA) on proliferation of rat colonic cells.

METHODS: EDTA was administered into Wistar rats, carcinogenesis induced by 1,2-dimethylhydrazine (DMH) in rats was studied with immunohistochemistry.

RESULTS: Marked regional differences in cell proliferation were found in all groups. In EDTA-treated animals, total labelling indexes in both proximal (10.00 ± 0.44 vs 7.20 ± 0.45) and distal (11.05 ± 0.45 vs 8.65 ± 0.34) colon and proliferative zone size (21.67 ± 1.13 vs 16.75 ± 1.45, 27.73 ± 1.46 vs 21.74 ± 1.07) were significantly higher than that in normal controls (P < 0.05) and lower than that in DMH group (10.00 ± 0.44 vs 11.54 ± 0.45, 11.05 ± 0.45 vs 13.13 ± 0.46, 21.67 ± 1.13 vs 35.52 ± 1.58, 27.73 ± 1.46 vs 39.61 ± 1.32, P < 0.05). Cumulative frequency distributions showed a shift of the EDTA distal curve to the right (P < 0.05) while the EDTA proximal curve did not change compared to normal controls. Despite the changes of proliferative parameters, tumours did not develop in EDTA treated animals.

CONCLUSION: Hyperproliferation appears to be more easily induced by EDTA in distal colon than in proximal colon. Hyperproliferation may need to exceed a threshold to develop colonic tumours. EDTA may work as a co-factor in colonic tumorigenesis.

Initiated stem cells in murine intestinal carcinogenesis: prolonged survival, control by NK cells, and progression

Weekly injections of dimethylhydrazine (DMH) (25 mg/kg), or azoxymethane (AOM) (8 mg/kg) to young adult male CDI mice for 1-2 months produced generalized intestinal crypt hyperplasia, which we measured in duodenum in terms of number of interphase and mitotic cells present in crypts. As shown earlier, the crypts expanded because of the presence of a hyperproliferative "initiated" crypt subpopulation which was also sensitive to natural killer (NK) cells. Hyperplasia was thus present as long as NK activity was suppressed by the carcinogen treatment. After interruption of the treatment for periods of 1, 2, 3, 6 and 10 months in the various groups, hyperplasia soon regressed as a result of elimination of the subpopulation by the recovering NK cells. When NK activity was once again eliminated during the terminal days of these "interruption periods" (by injections of anti-asialo GM-I antibody, alpha AGM-I), the original hyperplasia was fully reconstituted, apparently from stem cells of the subpopulation which survived up to 10 months in their crypt base location. These "initiated stem cells" represented, then, the original carcinogenic insult during the pre-cancerous period. They also appeared to be the source of the eventual neoplasia, as treating the animals with mutagens during the interruption periods produced specific changes in crypt base histology: new "crypt base basophilic" (CBB) cells appeared which produced large accumulations as well as microscopic tumors when NK activity was suppressed (by alpha AGM-I). Some of the initiated stem cells were apparently transformed into neoplastic ones which remained under NK control, the NK cells preventing the establishment of their progeny. Further experiments indicated that, although the initiated stem cells are not eliminated by normal NK activity, activated NK cells can kill them, thereby eliminating the potential source of neoplasia.

Hyperplasia of mouse duodenal crypts and its control by NK cells during the initial phase of DMH carcinogenesis

The possible regulatory role of NK cells on early events in chemical carcinogenesis remains undefined. The present study examined whether NK cells control 1,2-dimethylhydrazine (DMH)-induced hyperplasia of the duodenal crypt in CD1 mice. Mice receiving chronic DMH treatment showed a dose-dependent hyperplasia confined to the proliferative zone, with a parallel increase in mitotic and 3H-TdR-labelled cells and significant suppression of splenic NK activity. Complete ablation of splenic NK activity with anti-asialo GM-I antibody (alpha AGM-I) treatment slightly enhanced hyperplasia. Halving of the DMH dose for 2 weeks led to regression of hyperplasia, which was totally prevented by alpha AGM-I treatment. The alpha AGM-I treatment alone did not influence crypt size in normal mice. Finally, a stimulation of NK activity with Poly I:C treatment in DMH-treated mice caused regression of the DMH-induced hyperplasia. Our results suggest that hyperplastic cells with possible genetic alterations induced by the carcinogen express target structures for NK cells, but that simultaneous carcinogen-induced suppression of NK activity hampers their containment, allowing progression of hyperplasia to neoplasia, possibly owing to additional genetic changes.

Evidence for cell generation controlled proliferation in the small intestinal crypt

In this paper we discuss the hypothesis that cell proliferation is controlled by the number of generations after leaving an 'eternal' stem cell. The theory is based on a simulation of the kinetic behaviour of cells in the intestinal crypts. There is evidence of three, four and five generations of cells which are allowed to enter mitosis in the lower and upper part of the normal intestinal tract, and in some disease states, respectively. We suggest an internal proliferation control: some kind of knowledge that cells carry from generation to generation. It is an open question what sets and changes the generation counter: internal genetic information or external influences such as growth factors or chalones. The geometric shape of the epithelial tissue in the intestinal tract can be understood as the steady state of a highly dynamic process. Age and death are determined from the beginning; cell-cell interaction or communication is not necessary and can be neglected. Our theory will be illustrated using the intestinal crypts as they are easily accessible, of a simple structure and completely described in the literature.

Transplantation of a segment of ileum to the external abdominal wall: an animal model of intestinal mucosal hyperplasia

When a segment of small intestine is transplanted to the external abdominal wall in rats adaptive changes occur in the exposed mucosa. These probably represent an extreme example of a physiological response to one type of trophic influence--the effect of mechanical trauma. The nature of the changes has been studied at 7 weeks after externalization using simple morphometry and a number of cytokinetic techniques (thymidine labelling, vincristine-induced metaphase arrest and the fraction-of-labelled-mitoses method), and comparisons drawn with the normal ileum. The exteriorized mucosa showed marked villus atrophy and hyperplasia of the crypts to three times normal size as a result of increases both in crypt length and crypt circumference. Neither metaplastic nor dysplastic epithelial abnormalities were observed. Crypt-cell production rate doubled in the hyperplastic crypts due to an increase in the size of the proliferation zone within the crypt, and the distribution of proliferating cells within the crypt changed. But cell cycle times were prolonged and more maturing cells were retained in the hyperplastic crypts. The potential usefulness of this model, particularly in carcinogenicity studies is considered.

Are there diurnal fluctuations in crypt length and crypt cell birth rate in the intestines of normal and carcinogen-treated rats?

At eight time points over a period of 24 hours, crypt length (in cells) and crypt cell birth rate were measured by the stathmokinetic method with vincristine and evaluated at two sites in the small intestine and two in the colon in Wistar Porton rats. Normal animals were compared with animals which had received 24 injections, at weekly intervals, of the intestinal carcinogen 1,2-dimethylhydrazine. In the treated animals, crypts were significantly longer at sites prone to tumour formation but not at those sites known to be resistant to the effects of the carcinogen. In neither group of animals was there any significant fluctuation in mean crypt length over a 24 hours period. Crypt cell birth rates showed a considerable fluctuation. No difference was noted between normal animals and those treated with dimethylhydrazine and statistical analysis failed to confirm the presence of any true periodic fluctuation.

Morphological studies on the long-term organ culture of colonic mucosa from normal and dimethylhydrazine treated rats

Mucosal explants were prepared from the colons of normal rats and from the non-neoplastic colonic mucosa of rats which had been treated chronically with the intestinal carcinogen dimethylhydrazine. They were maintained in an organ culture system which permitted survival up to at least 25 days. Morphological preservation of the mucosa was excellent up to 6 days in culture and thereafter changes began to occur. But even at 25 days normal crypt structures were still evident. The hyperplastic and dysplastic changes seen in pre-culture samples of DMH-treated mucosae remained recognisable during the first two days in culture. They were no longer seen in explants examined after this time however and, indeed, there appeared to be no difference in the morphology and survival of control and DMH-treated mucosae. It is possible that our culture system does not permit further neoplastic progression, but an alternative explanation is that the system discriminates specifically against the survival of neoplastic elements.

Selective inhibition of replicative and repair DNA synthesis in mouse colon following administration of 1,2-dimethylhydrazine

Early alterations in normal semiconservative and repair DNA synthesis were determined in gastrointestinal tissues of HalCR mice following administration of the colon carcinogen 1,2-dimethylhydrazine (DMH). Following DMH injections of 60 and 200 mg/kg, normal DNA synthesis was rapidly inhibited in all tissues. The greatest depressions were observed in the descending colon, followed closely by the ascending colon. DNA repair was estimated by measuring unscheduled DNA synthesis. No repair was observable in the descending or ascending colon. The esophagus, forestomach, jejunum, and ileum demonstrated significant amounts of DNA repair, while the duodenum and gastric stomach displayed nominal or insignificant amounts of repair. Repair DNA synthesis was inhibited by simultaneous administration of caffeine and DMH. The degree of inhibition of normal replicative DNA synthesis and the amount of repair DNA synthesis in response to DMH treatment correlate closely with the incidence of DMH-induced tumors. Most tumors occur in the descending colon, followed by the ascending colon, and only a few in the duodenum and gastric stomach area. Neoplasms are rarely found in the remainder of the gastrointestinal system.

Study of kinetics of epithelial cell populations in normal tissues of the rat's intestines and in carcinogenesis. III. Changes in kinetics of enterocyte populations in the course of experimental intestinal tumour induction in rats

A stage-by-stage study of disturbances in enterocyte proliferation in the ileum and descending colon in the course of tumour induction by treatment with 1,2-dimethylhydrazine was performed. Even at early stages, an expansion of the zone of epithelial cell proliferation in the crypts and migration of dividing cells as far as to the crypt mouth, which is a manifestation of enterocyte differentiation disturbances, were observed. Enterocytes of the crypts chiefly proliferated through a short cycle, the mean duration of which was slightly greater than in normal intestinal tissue. The reduced cell loss in the epithelium and resultant disturbances of its steady state led to the accumulation of great numbers of atypical cells in the superficial layers of the crypts and formation of carcinomas in situ in the descending colon. The microscopically unaltered sections of the mucosa, prior to development of overt neoplastic changes carcinomas in situ, superficial cancers and small-size adenocarcinomas revealed a simplified structure of enterocyte population, as compared with normal epithelium. As tumours progressed, the heterogeneity of its component cell subpopulations increased, and several subpopulations, differing in mean duration of the mitotic cycle, were formed. Pathologic mitoses made up a greater portion (50-60 per cent) of the dividing cells of the descending colon, as compared with ordinary 4 per cent at all stages of experimental tumour induction.

Acute changes occurring in the intestinal mucosae of rats given a single injection of 1,2 dimethylhydrazine

In the long term, administration of dimethylhydrazine (DMH) to rats results in the development of tumours in both small intestine and colon. This study has been undertaken in order to document the sequence of changes occurring in the intestinal mucosa in the first 108 h following a single subcutaneous injection of DMH. After a lag of several hours there is evidence of damage to cells in the proliferation zone of the intestinal crypts, and a brief reduction in tritiated thymidine labelling index. A phase of compensatory regenerative activity emerges from the setting of continuing cell damage, resulting in restoration of the mucosa to normal. The severity of the toxic damage to the intestinal mucosa at various sites mirrors the vulnerability of the mucosa to the long term carcinogenic effects of DMH, suggesting that inherent properties of the mucosa may be of more importance than other cocarcinogenic influences in the ultimate development of tumours.

Kinetics of the non-neoplastic mucosa of the large bowel of dimethylhydrazine-treated rats

Administration of 1,2 dimethylhydrazine (DMH) to rats by weekly s.c. injections causes the development of multiple epithelial tumours of the large bowel. These appear to arise as localized dysplastic abnormalities in hitherto apparently morphologically normal crypts. This study was undertaken in order to examine cell proliferation in such apparently normal crypts of DMH-treated animals. A number of proliferative abnormalities are evident, including changes in the size of the crypts, changes in the disposition of proliferating cells within them and reduced cell-cycle times. The nature and the extent of the abnormalities vary from site to site along the length of the bowel, and reflect the vulnerability of the different segments of the bowel, not only to the carcinogenic effects of DMH, but also to short-term toxicity.

Experimental adenomas and carcinomas of the large intestine behave as distinct entities: most carcinomas arise de novo in flat mucosa

Detailed histologic analyses were performed on tumors of the large intestine obtained in 152 dimethylhydrazine (DMH)-treated rats. Of a total 539 glandular neoplasms, 45 were benign; 494 (92%) were locally invasive; of which 222 (41%) were invading the muscularis propria. One-hundred-forty-one tumors were smaller than or equal to 3mm in diameter. Among those, 127 (90%) were invasive. In addition to macroscopic nodules, several appeared after longer latency periods than did serial sections of flat mucosa. The benign polyps usually appeared after longer latency periods than did carcinomas. A review of the literature indicates that in the majority of rat experiments most or all DMH-induced tumors were frequently reported. All these data constitute strong evidence that most experimental adenocarcinomas do arise de novo in flat mucosa, i.e., without a prior adenoma stage. However, most DMH-induced tumors in mice were reported to be adenomas, either alone or coexisting with carcinomas. It is suggested that "de novo arising carcinomas" and adenomatous polyps, which are both inducible by the same carcinogens, and which can coexist in some experimental systems, nonetheless constitute independent and distinct pathologic entities; they can be separated by genetic susceptibility.

Cell proliferation of colonic neoplasms in dimethylhydrazine-treated rats

We have measured mitotic indices and 3H-thymidine-labelling indices for the colonic epithelial tumours induced in rats by the administration of dimethyl-hydrazine (DMH). The fraction-of-labelled-mitoses (FLM) technique has been used to estimate the duration of the cell-cycle phases. In general, mitotic and labelling indices in the tumours are similar to those in the proliferation zone of the normal crypt epithelium; lesions considered to be least well differentiated on histological grounds appear to have the lowest mean labelling index. Benign tumours and the different types of malignant tumours have mean cell-cycle times about half those of the normal mucosa.

Intracellular adenosine and guanosine 3',5'-cyclic monophosphate concentrations in rat small and large bowel following single and multiple exposures to 1,2-dimethylhydrazine

The adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) levels were determined in the small and large intestinal tissue of rats that had been exposed to single and chronic administration of the colon carcinogen 1,2-dimethylhydrazine (DMH). A single subcutaneous injection of DMH resulted in a decrease in the intracellular concentration of cAMP and increase in cGMP beyond the levels which had been measured in the unexposed intestinal tissue and DMH induced intestinal adenocarcinomas. Recovery to normal concentrations of the cyclic nucleotides occurred within 30 days. Multiple exposures resulted in maintaining reduced levels of cAMP while cGMP was also found to be lowered upon the chronic administration. A possible explanation for these observations is the expansion of the crypt cell population consisting of replicating intestinal cells that occurs upon exposure to the carcinogen. These findings suggest that cyclic nucleotide alterations may represent a characteristic of the precancerous state of intestinal tissue and indicates further studies are warranted to determine whether these changes may serve as a useful marker in a screening program for colon cancer.